The polarizing plate having light transmission/shielding function and also a liquid crystal having light switching function are fundamental components of display devices such as liquid crystal displays (Liquid Crystal Display: LCD). The application field of this LCD has been also extended from small instruments such as initial electronic calculators and clocks to laptop personal computers, word processors, liquid crystal projectors, liquid crystal televisions, car navigation systems and indoor-outdoor measurement instruments, and the like. In addition, it can be also applied to a lens having polarization function, and has been applied to sunglasses with improved visibility and also, in recent years, polarized glasses for 3D televisions, and the like. As above, the application of the polarizing plate is widely broadened and it is used under broad conditions from low temperature to high temperature, from low humidity to high humidity, and from low light amount to high light amount, so a polarizing plate having high polarization performance and excellent durability is required.
At present, the polarizing film is manufactured by dyeing, with iodine or a dichroic dye as a dichroic coloring matter, a polarizing raw film such as a film formed by stretching and orienting polyvinyl alcohol or a derivative thereof or a polyene-based film formed by producing a polyene by dehydrochlorination of a polyvinyl chloride film or by dehydration of a polyvinyl alcohol-based film and then orienting the polyene, or manufactured by allowing iodine or a dichroic dye as a dichroic coloring matter to be contained in the polarizing raw base. Among them, the iodine-based polarizing film using iodine as a dichroic coloring matter has excellent polarization performance but is vulnerable to water and heat, and thus it has a problem with its durability when used in the state of high temperature and high humidity for a long period of time. Some solution methods have been considered, such as treatment with formalin or an aqueous solution containing boric acid and use of a polymer film having a low water-vapor transmission ratio as a protective film, in order to improve durability, but their effects are not sufficient. On the other hand, the dye-based polarizing film using a dichroic dye as a dichroic coloring matter is excellent in moisture fastness and heat fastness as compared with the iodine-based polarizing film but generally has insufficient polarization performance.
In a neutral-color polarizing film formed by adsorption of a few kinds of dichroic dyes into a polymer film and by orientation, if light-leakage (color-leakage) of a certain wavelength in a wavelength region of a visible light region occurs in the state where two polarizing films are superposes so that their orientation direction are orthogonal (in the orthogonal position), the hue of a liquid crystal display could change in a dark state when the polarizing film is provided on a liquid crystal panel. In order to prevent discoloration of a liquid crystal display due to color leakage in a certain wavelength in the dark state when a liquid crystal display is provided with a polarizing film, the transmittance ratio in the orthogonal position (orthogonal transmittance ratio) in the wavelength region of the visible light region must be uniformly reduced in a neutral color polarizing film formed by adsorption of a dichroic dye into a plural kinds of polymer films and by orientation.
In the case of a color liquid crystal projection-type display, i.e., a color liquid crystal projector, a polarizing plate is used in its liquid crystal image formation part. Previously, an iodine-based polarizing plate having good polarization performance and exhibiting neutral gray was used. However, the iodine-based polarizing plate has a problem of insufficiency in light fastness, heat fastness and wet-heat fastness because iodine is used as a dichroic coloring matter as described above. In order to solve this problem, a neutral gray polarizing plate with a dye-based dichroic coloring matter being a polarizer has been increasingly used. For this neutral-gray polarizing plate, coloring matters of three primary colors are usually used in combination in order to averagely improve the transmittance in all visible light wavelength regions and polarization performance. For this reason, a neutral gray polarizing plate using a dichroic dye as a dichroic coloring matter does not have sufficient light transmittance, so there is a problem that the light source intensity must be higher in order to meet market needs of a brighter projection image in a color liquid crystal projector. In order to solve this problem, three color dye-based polarizing plates corresponding to three primary colors, i.e., for blue channel, green channel and red channel, have been used instead of the neutral gray polarizing plate.
However, reduction in brightness cannot be avoided due to considerable absorption of light by a polarizing plate, enlargement of an image having a small area of 0.5-3 inches to a few dozen-hundred inches, and so on, and therefore as its light source, one having higher luminance is used. Additionally, further improvement of brightness of a liquid crystal projector is firmly required, and as a result, intensity of a light source used has been increasingly enhanced, which also involves increase in light and heat on a polarizing plate.
The dye used for manufacturing the above-described dye-based polarizing film includes, for example, water-soluble azo compounds described in Patent Literature 1 to Patent Literature 7, and the like.
However, conventional polarizing plates containing the above water-soluble dye have yet to sufficiently satisfy market needs from the viewpoint of polarization properties, absorption in wavelength region, hue and the like. In addition, for the three color dye-based polarizing plates for blue channel, green channel and red channel corresponding to three primary colors for a color liquid crystal projector, there is no polarizing plate having brightness and polarization performance and being good in all of durability under high temperature and high humidity conditions and also fastness to light exposure for a long period of time, so improvement of conventional polarizing plates is desired.    Patent Literature 1: Japanese Patent No, 2622748 A    Patent Literature 2: Japanese Patent Laid-Open No, 2001-33627 A    Patent Literature 3: Japanese Patent Laid-Open No. 2004-51645 A    Patent Literature 4: WO2005/075572 A    Patent Literature 5: WO2007/148757 A    Patent Literature 6: Japanese Patent Laid-Open No. 2003-327858 A    Patent Literature 7: Japanese Patent Laid-Open No. 2005-255846 A    Patent Literature 8: Japanese Patent Laid-Open No. 2004-075719 A    Non-Patent Literature 1: Dye Chemistry; written by Yutaka Hosoda